Heterobihalide ions. Nuclear magnetic resonance spectroscopy of strong hydrogen bonds

Abstract

The hydrogen and fluorine nmr spectra of the fluorine bihalide ions FHX~( X = F, Cl, Br, and I) complete our study of the ten bihalide ions in aprotic solvents. The H-F coupling constant has been resolved in all four fluorine bihalides. Exchange of fluorine is slow; exchange of the other halogens is fast on the nmr time scale. The formation and interconversion equilibria may be adequately interpreted by assuming quantitative hydrogen transfer to the most basic (the smallest) halide ion in the system. Consistent with this, we find that the predominant complex in a solution of HC1 and excess Bris ClHBrnot BrHBras previously reported (Fujiwara and Martin, J. Chem. Phys., 56, 4091 (1972)); the proton shieldings in these ions are coincidentally the same. A heterobihalide ion XHYbehaves as an anion-molecule H-bonded complex XH • • • Y-, where is less than . A simple electrostatic interpretation of the charge shifts on H bonding appears adequate to account for the observed formation constants and hydrogen shieldings. It is possible to make predictions of the bond lengths

Strong hydrogen bonding in oxime solvates of tetra-n-butylammonium fluoride

“…This is not without precedent for fluorides since it has been reported that crystals of KF(CH,CO,H), grow from KF-CH,CO,H solutions (1). All of the disolvates are soluble in common polar aprotic (2,4,7). The commonly observed effect on the H-bonding proton resonance on going from the self-associated electron acceptor to the anionic complex is a deshielding (downfield shift) one which is usually assumed to reflect an increase in strength of the H-bond.…”

“…It would seem, therefore, that while the intermolecular association of this oxime is disrupted by the highly localised electron density of the fluoride anion, the intramolecular association remains intact (eq. [4]). …”

“…In recent years, a number of complexes of the type AH...B-where AH is an electron acceptor hydrogen bonded to an electron donor anion B-, have been studied by a wide variety of spectroscopic and nonspectroscopic techniques (1)(2)(3)(4)(5)(6). When AH is a strong I H-bond electron acceptor such as a carboxylic acid or phenol and B a strong H-bond electron donor such as a small halide or carboxylate anion, the resulting complex is unusually stable and the spectro-( scopic and nonspectroscopic manifestations of H-bonding are exaggerated providing the opportunity for more detailed and more informative investigations of the H-bonding phenomenon.…”

Nuclear Scalar Spin-Spin Coupling Reveals Novel Properties of Low-Barrier Hydrogen Bonds in a Polar Environment

“…In Figure 1 (Figure 1b) and 19 F NMR (Figure 1c). For the former sample b was used, containing a higher total concentration of the complex and a smaller excess (7-fold) of the base.…”

Direct preparation of bromoacetaldehyde

“…[17][18][19][20] In additional experiments with TBAF trihydrate a t 77 "C (2 torr) we noted not only mass loss as a function of time but also changes in physical appearance as well. For example, when placed at 77 OC, the crystalline trihydrate immediately melted to a colorless liquid from which bubbles evolved.…”